This invention relates to processing machines with rolls in general and more particularly to an improved pressure control device for processing machines having at least one roll which has a stationary core, around which a hollow roll revolves which forms the roll proper, radially spaced from the core, and supported directly on the core at its ends and supported in the radial direction in between the ends through a fluid pressure medium which is disposed in a chamber formed at the core.
In such a device the pressure medium generates a force acting on the inside of the hollow roll and, thereby, controls the operating pressure of the roll. Such processing machines typically have a loading device which is operated by a fluid pressure medium and applies a force at the roll or a counter roll. In this way the loading device or the roll form the controlling element and there is derived from the pressure prevailing therein a control pressure, by means of which, in the respective other controlled element a pressure can be adjusted which is in a definite, predetermined pressure ratio to the pressure in the controllable element. The invention is applicable particularly to processing machines, in which the loading device is the controlling element.
From U.S. Pat. No. 2,908,964, an embodiment of such a pressure control device for a pair of rolls is known. In the disclosed device the rolls are designed as so-called "swimming rolls". In such rolls, a seal extends along the core between the latter and the inside of the hollow roll. This seal together with transversal seals provided at the ends of the hollow roll, subdivides the space between the core and the hollow roll into two lengthwise chambers which are filled with hydraulic oil which constitutes the fluid pressure medium. The lengthwise chamber situated on the side of the roll gap receives the higher pressure. The pressure difference determines the working pressure, i.e., the line pressure in the roll gap. The hydraulic oil acts directly against the inside of the hollow roll.
The present invention, however, is also applicable to other types of rolls in which pressure is transmitted to the inside of a hollow roll by mechanical intermediate members such as pressure shoes or roller arrangements. Such rolls are described, for instance, in U.S. Pat. No. 2,395,915, German Offenlegungsschrift No. 2 230 139 and German Auslegeschriften Nos. 1 193 792 and 1 561 706.
In the device described in U.S. Pat. No. 2,908,964, one of the so-called "swimming rolls" is fixedly supported, while a loading device which comprises two hydraulic piston/cylinder units, which transmits their force to the ends of the core of the counter roll, acts, in the case of the other roll, on the ends of the core which form the journals of the roll and protrude from the hollow roll.
The important point is that the pressures in the floating rolls and in the loading device are very accurately matched to each other. If, for instance, the pressure in a floating roll is too high, i.e., higher than the counter forces require, then the floating roll will be bent, since its hollow roll is supported on the core at the ends and the excessive pressure in the center strives to push the hollow roll away from the core toward the counter roll or, in any event, will cause a non-uniform line pressure in the roll gap. All this is true not only for the so-called "swimming roll" but also for the other types of rolls known from the above-mentioned references.
It is a further disadvantage, if the pressure in the rolls is not accurately matched to the situation, that the bearings at the ends of the hollow roll are then subject to an excessively high load. In the ideal case, i.e., if the pressure in the roll is accurately matched, the counterforce should be in equilibrium with the force exerted by the fluid pressure medium and the bearings at the end will have only a guiding function. But if the pressures do not balance, then the bearings must take up the pressure difference, which leads to an unbalanced force between the hollow roll and the core. The same is true if the force of the loading device does not exactly correspond to the pressure in the roll. If, for instance, a very low pressure in the roll occurs at the same time as a very high pressure at the loading device, the forces exerted thereby are fully conducted onto the bearings in the roll.
For monitoring the pressure ratio in the roll and in the loading device, a pressure ratio control is provided in the embodiment described in U.S. Pat. No. 2,908,964. This pressure ratio control regulates the pressure at the loading device so that the force exerted by a web of material passing between the rolls is just cancelled by that pressure due to the hydraulic oil in the lengthwise chamber of the "swimming roll". Then, all pressure is transmitted by the hydraulic oil, while the bearings at the ends of the hollow roll remain free of the forces acting in the roll gap. In the embodiment described in U.S. Pat. No. 2,908,964, the controlling variable is the pressure in the "swimming roll".
However, it is also well known in the art to provide the control in such a processing machine in the inverse manner, i.e., to choose the pressure in the loading device as the controlling variable and to set the pressure in the roll accordingly.
It is a condition for the functioning of such a pressure control device that the pressure in the controlled element can actually reach the pressure which the controlling element demands. However, this is not always the case in practice; rather, the pressure in the controlled element sometimes cannot follow the controlling variable fast enough or cannot follow it at all, be it because of a lack or excessive sluggishness of the pressure supply or because of pressure losses occurring at certain points.
In a processing machine with rolls of the type in question, in which the pressure in the loading device is the control input, this can occur in two cases in particular. First, it occurs if the loading pressure rises faster than the pressure in the roll can follow. Such an increase of the loading pressure is necessary if, after inserting a web into the processing machine, the line pressure must be increased to the full operating pressure. Secondly, the desired pressure ratio is not obtained if the pressure in the roll does not ever come up to its normal magnitude for whatever reason. This may be due to an insufficient capacity of the pump system, continuous leakage losses because of a faulty condition of the seals in the roll, defective connections in the control lines, etc.
Similar problems can occur if the pressure in the roll is the controlling input.